As a boomkin, altho the 2p reads to be really nice, I know boomers who are saying the class feels clunky without the 2sec dots from the previous 4p. So I pretty much want all or nothing. Going straight from 4p to 4p would suit me nicely, but it depends if it is really viable in your raid to give one person all 4 pieces before someone else gets 2..

For a hc guild puching undergeared progression, you'd want dps first as a lot of people have said.

For a guild who gets to bosses and has enough gear for enrages to not be an issue I would say either tanks or healers and then dps, but NEVER tank AND healer before dps. Reason for this is that gearing up the tank and gearing up the healer basically does the same thing. Stops the tank from dying.
Other than keeping the tank alive, gearing up healers first will keep the raid alive and gearing up tank will increce raid dps. It depends on what your group needs.

As a boomkin, altho the 2p reads to be really nice, I know boomers who are saying the class feels clunky without the 2sec dots from the previous 4p. So I pretty much want all or nothing. Going straight from 4p to 4p would suit me nicely, but it depends if it is really viable in your raid to give one person all 4 pieces before someone else gets 2..

Well, I went from T14 4p -> T15 2p & T14 2p -> T15 4p and it felt quite a bit clunky at the beginning, but the additional crit (both and bonus) kinda made up for that...and I got T15 4p the next day and you can get some badass SS procs, so it's worth it. You may consider switching back to T14 4p on heavy multidotting bosses before you get your T15 4p though. Pretty happy that I got all the tokens and got my 4p when max 3 other DDs "only" had their 2p, so I don't have to worry about it now :P But the run on the Druid Mage Rogue DK wasn't big, their bonuses seem to be kind of lame...

There are some cases where tank set bonuses are really worth it, such as the T14 10% increase to SOTR damage reduction for prot paladin. There are some cases where healer bonuses are really worth it, such as t14 holy paladin. Set bonuses are very hit or miss, both for a guild and for the individual. This tier, the holy paladin 4p bonus is quite bad. Not because it can't increase your throughput, but because that throughput is limited to your beacon target. Reducing the holy shock CD (t14 bonus) gave holy a hell of a lot more flexibility in addition to considerable throughput benefits, especially for AoE.

The best guilds in the world have the best tanks and healers in the world, and given some practice they WILL overcome the healing requirements of any encounter. DPS on the other hand is MUCH more gear-dependent. That doesn't mean that us normal people should follow their example. I think for the average group, healing gear is going to be the most important. Even in t15 normal progression, a lot guilds 'brute-forced' their way through the encounters because their healers are very good, and generally overgeared for the content. Going through normals at ilvl 494 as holy, I can definitely imagine how much easier it would be if I had the same 510+ gear that most heroic raiding healers had by 5.2

To say for the dk's, I know that the 2 piece on dps is pretty meh, I mean it's a pretty easy proc and adds nice dmg but it's passive so not mind-blowing awesome. However the 4 piece is amazing imo. Able to use your execute move from 45% and lower? That's awesome, we already had the largest execute range being it standard at 35% but having it for almost half the fight is great. (this is from and Unhoyl dk's standpoint, I know being able to use KM procs on SR is a diff story maybe?)
Also the point for soloing content and being able to be in blood yet benefit from the 2 and 4 set is great as well ^^

The 2 set for Blood is pretty nice, no cost on RT and 10 sec lower CD, meaning that this lines up with a 40 sec VB from the T14 2 set very nicely. Overall the 2 set is awesome, however then free RP from expending bone shield charges, is just lackluster.